The conventional method of anodising aluminum to achieve the characteristic properties and appearance on the surface of the metal is to make the aluminum article the anode in an electrolytic bath having sulphuric acid as electrolyte, for example at a concentration of about 16% by weight of acid in water, and pass an electric current between the cathode and the anodic workpiece. By anodising aluminum in this way, oxide coatings of sufficient thickness to satisfy specifications for the use of aluminum in external architectural constructions are readily achieved. However, anodised surfaces prepared by the sulphuric acid route suffer from the disadvantage that they are susceptible to attack by alkaline media which stain the surface over a short period of time. It is therefore necessary in the building industry to protect all anodised aluminum construction, typically door frames, window frames and decorative cladding, from contact with alkaline materials, such as wet mortar. At present, the surfaces are usually protected in the United Kingdom by the use of adhesive tape over the significant surfaces, which is a time consuming procedure done by hand. In practice on a building site, the adhesive tapes may peel, leaving exposed surfaces susceptible to staining by any mortar which may fall or splash. Furthermore, after the building is completed, the protective tape must be removed, but the mortar may still be green and rain will leach alkali which could contact the surface of the anodised aluminum and cause staining.
Alternative proposals for protecting anodised surfaces have included lacquering followed by baking and the use of air drying lacquers or waxed coatings, but these techniques have not been considered favorably in the United Kingdom. In some instances these coatings interfere with the adhesion to the anodised surface of the mastic compounds used in the building industry.
Other techniques for anodising aluminum have been proposed using alkaline electrolytes. British Pat. No. 1,243,741 discloses the use of alkaline solutions of alkali metal or ammonium borate as electrolytes. Coatings were obtained which had similar properties to anodic coatings of similar thickness produced by conventional sulphuric acid processes, but processes using alkaline electrolytes have not been acceptable for external architectural work in general as it has not been possible hitherto to produce anodic coatings of sufficient thickness. For example, British Standard BS3987: 1974 states that an anodic coating of a thickness of at least 25 .mu.m is required on the surface of aluminum for external architectural use. It has not been found to be practicable to form coatings approaching this value using alkaline electrolytes.
It is therefore desirable to have external architecture manufactured from anodised aluminum with the property of being resistant to attack from alkaline media which would render the use of adhesive tape and other protective devices unnecessary for this purpose.
Thus, an object of the present invention is to provide a method of anodising aluminum so as to form thereon an oxide coating which is of sufficient thickness to comply with normal specifications for external architectural use and which is also more resistant to staining by alkaline media.
A second object of the invention is to provide a more stain resistant anodised aluminum article which retains the desirable properties of aluminum anodised by any of the conventional acid processes.